1. Field of the Invention
The present invention relates to a method for producing superconducting thick films, and more particularly, to a method for producing a superconducting thick film used, for example, for a dielectric resonator.
2. Description of the Related Art
Because of the recent rapid propagation of mobile communication, a shortage of radio-wave frequencies used for communication is anticipated in the near future. Therefore, in order to effectively use the limited frequencies, the development of a microwave filter used in base stations, having low loss and steep attenuation characteristics, has been desired. As such a filter, for example, a dielectric resonator is used. An example of such a dielectric resonator is a TE.sub.011 -mode dielectric resonator as shown in FIG. 1. The dielectric resonator 10 includes a copper plate 12. A dielectric 14 is placed on the copper plate 12. A substrate 18 provided with a superconducting film 16 is further placed on the dielectric 14. Therefore, the dielectric 14 is sandwiched between the copper plate 12 and the superconducting film 16. Excitation cables 20 and 22 are disposed so as to be opposed to each other on both sides of the dielectric 14 between the copper plate 12 and the superconducting film 16.
Another example is a TM.sub.010 -mode dielectric resonator as shown in FIG. 2. A dielectric resonator 10 includes a dielectric substrate 30 and superconducting films 32 and 34 formed on both sides of the dielectric substrate 30. The dielectric substrate 30 is fixed within a metallic case 38 with a Teflon sheet 36 therebetween. The metallic case 38 is provided with an excitation cable 40 on one end and an excitation cable 42 on another end.
Such a dielectric resonator 10 uses a phenomenon in which an electromagnetic wavelength is shortened to 1/(.epsilon.r).sup.1/2 (where .epsilon.r is the relative dielectric constant) in the dielectric in comparison with that in free-space, and is used in various resonant modes such as a TE mode, TM mode or TEM mode. In such a dielectric resonator 10, its unloaded Q (Qu) depends on both dielectric Q (Qd=1/tan .delta.) and Q (Qc) due to conductor loss resulting from an electric current on the surface of the metal, and Qu is expressed by the following equation: EQU 1/Qu=(1/Qd)+(1/Qc)
Thus, in order to obtain a resonator having high unloaded Q (Qu), a dielectric material having high Qd as well as electrodes having high Qc, i.e., a low conductor loss, must be used. Accordingly, as shown in FIG. 1 or FIG. 2, superconducting films 18, 32 and 34, having smaller surface resistance than that of a conductive metal such as copper, are used, whereby conductor loss can be reduced. with respect to such superconducting films 18, 32 and 34, various investigations and developments have been made mainly on yttrium-based thin films.
However, in view of implementing the industrial use in the case of thin films, production costs are significantly high, and it is difficult to form thin films having large areas. On the other hand, in the case of thick films formed by screen-printing or the like, although production costs are significantly low in comparison with the thin film process and it is easy to form large areas, the surface resistance is large because the surface state and the grain orientation of thick films are inferior to thin films. In particular, with respect to a Bi-based 2223 phase which has a high critical temperature Tc (110.degree. K) among oxide high temperature superconductors and the implementation of use of which is expected, the surface state deteriorates because flaky grains grow in a disorderly way, and improvement of the grain orientation and the surface state has been required.